1. Field of the Invention
The present invention relates to a dispersion-type liquid crystal electro-optical device based on a liquid crystal-resin composite comprising a polymer resin having dispersed therein a liquid crystal material, or on a liquid crystal-resin composite comprising a liquid crystal material having dispersed therein a resin. The present invention also relates to a method for forming the same.
2. Prior Art
Many liquid crystal electro-optical devices operating on a twisted nematic (TN) or a super-twisted nematic (STN) mode using nematic liquid crystal and the like are known and put into practice. Recently, there is also known a device using ferroelectric liquid crystals. All those liquid crystal electro-optical devices basically comprise a first and a second substrate each having provided thereon an electrode and a lead, and a liquid crystal composition being incorporated between those facing two substrates. Thus, by applying an electric field between the electrodes, the state of the liquid crystal molecules having incorporated between the substrates are changed according to the anisotropy in dielectric constant. Otherwise, in the case of ferroelectric liquid crystals, the state of the liquid crystal molecules are changed according to the spontaneous polarization of the liquid crystal molecule itself. The liquid crystal devices take advantage of this electro-optical effect for displaying images.
In a liquid crystal operating in a TN or an STN mode, the liquid crystal molecules which are brought into contact with the two substrates are subjected to an orientation treatment and are arranged along the rubbing direction. The rubbing directions in the upper and the lower substrates are twisted with respect to each other so that they may make right angle or an angle from 200.degree. to 290.degree.. Thus, it can be seen that the liquid crystal molecules at the midway between the two substrates are arranged in spirals to achieve a minimum energy. In the case of an STN type device, a chiral substance is added to the liquid crystal material if necessary.
The devices described hereinbefore, however, require essentially a polarizer sheet. Furthermore, the liquid crystals must be arranged along one direction within the liquid crystal electro-optical device. Such a regular orientation of liquid crystal molecules had been achieved by rubbing an orientation film (generally an organic film) with a cotton or a velvet cloth. If not for such an orientation treatment, the liquid crystal molecules would not arrange themselves along one direction and the electro-optical effect of the liquid crystals would not be fully exhibited. Thus, a conventional device generally takes a cell like structure comprising a pair of substrates to support the liquid crystal material therebetween, so that the liquid crystal may be injected and then imparted orientation by applying a rubbing treatment thereto to exhibit the optical effect.
In addition to the liquid crystal electro-optical devices of the type above, there is also known a dispersion-type liquid crystal capable of providing a clear and high contrast image plane, yet free of such polarizer sheets and rubbing treatment mentioned above. A typical type of a prior art dispersion-type liquid crystal device comprises a light-transmitting solid polymer having dispersed therein granular or sponge-like liquid crystal materials to give a light-control layer. Another typical type of a prior art dispersion-type liquid crystal device comprises a liquid crystal material having dispersed therein a solid polymer with the liquid crystal molecules being oriented at random. This liquid crystal device can be fabricated by dispersing encapsulated liquid crystal materials into the polymer, and then coating a film or a substrate with the resulting polymer to give a thin film. Materials such as gum arabic, poly(vinyl alcohol), and gelatin can be used for encapsulating the liquid crystal materials.
Let us consider a case in which the microcapsules are prepared by encapsulating a liquid crystal material with poly(vinyl alcohol). If the liquid crystal molecules exhibit a positive dielectric anisotropy in the polymer thin film under an electric field, the molecules are arranged in such a manner that the major axes thereof are in parallel with the electric field. Thus, if the refractive index of the liquid crystal is the same as that of the polymer, the thin film turns transparent. When the applied electric field is removed, the liquid crystal molecules take a random orientation to hinder light path, and thus the film turns opaque. Various types of information can be displayed by taking advantage of the two states, i.e., a light-transmitting and an opaque state. Dispersion-type liquid crystals include, in addition to the encapsulated type above, those comprising liquid crystal materials being dispersed in an epoxy resin, those taking advantage of phase separation between liquid crystals and a resin by irradiating a light to a mixture of a liquid crystal and a photo-curable resin, and those comprising a three-dimensionally structured polymer having impregnated with a liquid crystal. The present invention refers to all those mentioned hereinbefore collectively as dispersion-type liquid crystals.
Because those dispersion type liquid crystal electro-optical devices can be fabricated free of polarizer sheets, an extremely high light transmittance can be achieved with the devices of this type as compared with the conventional ones operating in a TN or an STN mode and the like. More specifically, the transmittance per polarizer sheet is about 50%, and in a device driven by an active matrix using a plurality of polarizing sheets in combination, the transmittance falls to a mere 1%. The transmittance of an STN mode device also falls to about 20%, and hence efforts are made to obtain a brighter image plane by increasing luminance of the backlighting. In contrast to these devices, a dispersion-type liquid crystal electro-optical device transmits 50% or more of the light. This owes to the fact that the device can be made completely free of polarizers.
As described in the foregoing, a dispersion-type liquid crystal functions by changing its state, i.e., a transparent state and an opaque state, and is advantageous in that it allows transmission of light at a larger amount. Thus, R & D efforts are paid mainly in realizing a transmitting-type liquid crystal electro-optical device; more particularly, in the realization of a projection type liquid crystal electro-optical device. In a projection type liquid crystal electro-optical device, the light is passed through a liquid crystal electro-optical device panel established midway in the light path of an incident light from the light source, and the light having passed through this panel is projected onto a wall via a slit having a predetermined angle. The liquid crystal molecules in the panel are in a random arrangement to give an opaque state when a low electric field below a certain threshold value is applied, i.e., when a low voltage to which the liquid crystal molecules do not respond is applied. The light is scattered upon incidence to the panel at this state, thus enlarging the light path. Then, the slit provided next to the panel cuts off most of the scattered light to give a black state to the wall. On the other hand, a light incident to liquid crystal molecules having arranged in parallel in correspondence to the applied electric field passes straight through the molecules without being scattered, thereby yielding a light state on the wall at a high brightness.
It can be seen from the foregoing description that, in a dispersion-type liquid crystal electro-optical device, the degree of the light scattering in accordance with the change in molecular orientation of the liquid crystal determines the contrast of a display. Accordingly, the liquid crystal material inside the device must be in a finely dispersed state. The fine liquid crystal droplets in general are in the range of from 0.3 to 3 .mu.m, with maximum size being 10 .mu.m and minimum being 0.05 .mu.m. In case of too small liquid crystal droplets, the liquid crystal material greatly suffers disturbance exerted by the transparent resin material surrounding them. Then it becomes practically unfeasible to drive them with an external electric voltage conventionally applied in the art. Otherwise, the quantity of the liquid crystal material operable with a conventional driving power considerably decreases owing to such disturbance and again results in an insufficient realization of an electro-optical effect. In the latter case, a favorable contrast cannot be achieved.
Even in the states above, it is still possible to drive the liquid crystal materials by increasing the driving voltage. However, such a liquid crystal electro-optical device cannot be driven with a semiconductor integrated circuit (IC) because such a high driving voltage cannot be realized with an IC. In view of the fact that the maximum allowable driving voltage at present is about 30 V, there may be considered increasing the thickness of the liquid crystal layer as an alternative measure to achieve a high contrast. But again, the result is a further drop in contrast, because the output voltage from the semiconductor device cannot be increased and hence the applied intensity of the electric field becomes too low to drive a liquid crystal.